The present invention relates to a process for the detoxification of aqueous solutions containing cyanide and/or other oxidizable substances. The process utilizes a mixture of hydrogen peroxide or other peroxygen compounds and a source of silicon dioxide as catalyst.
Wastewater from a variety of industrial processes poses a serious problem to the environment because of the presence of components in the wastewater that are harmful to the environment. Effluents from a variety of industrial processes often contain cyanide in various forms, such as cyanide and cyanide complexed with other toxic metals or organic nitriles. The conversion of these toxic substances into nontoxic materials is of paramount importance before such wastewaters are discharged to the waterways in order to avoid serious damage to the environment and aquatic life, and to avoid an adverse impact on public health and safety.
It has been known for some time that one of the best known methods to destroy cyanide ions in aqueous wastewater solutions is by oxidizing it to harmless species with hydrogen peroxide. However, this process is usually fairly slow. Sometimes a catalyst is used, such as copper in the Cu.sup.2+ state, in order to accelerate the rate of the reaction (U.S. Pat. No. 3,617,567). U.S. Pat. Nos. 4,416,786 and 4,851,129 also focused on lowering the consumption of peroxide per amount of cyanide destroyed. U.S. Pat. No. 4,996,715 discloses the use of phosphoric acid in a mixture with peroxide to detoxify slurries and high solids suspensions and solutions. This also would lower the consumption of peroxide per amount of cyanide destroyed.
In many situations, the greatest limiting factor on the use of hydrogen peroxide for the detoxification of a given wastewater is the reaction time and the overall cyanide removal effectiveness. Copper does accelerate the rate of reaction as a catalyst, however it is undesirable to add copper to a wastewater stream because copper is a toxic heavy metal. Heavy metals are strictly regulated in wastewater discharge permits. They are quite toxic to aquatic organisms. They can sometimes be difficult to remove from solution. Even after removal, a sludge containing heavy metal has to be disposed of. Therefore, using them adds much additional cost to the wastewater treatment process. In addition, in some situations the use of copper does not allow the cyanide to be destroyed to sufficiently low levels as required by the wastewater discharge permit.
Several researchers have tested silica or silicon dioxide as a catalyst for the oxidation of a subject compound by hydrogen peroxide:
Hoss et al., "Sulfur (IV) Oxidation by Hydrogen Peroxide in Aqueous Suspensions of SiO.sub.2, Al.sub.2 O.sub.3, TiO.sub.2 and Zeolite", Atmospheric Environment (1991), Vol. 25A, No. 8, pp. 1715-1717, tested silicon dioxide or silica as a catalyst for the oxidation of sulfur IV to sulfur VI in their work. They found the reaction did not benefit from the presence of silicon dioxide, aluminum trioxide, titanium dioxide or type A zeolite.
Fischer and Hubert (U.S. Pat. No. 5,068,038) used silica to flocculate and adsorb organic halide pollutants after oxidation by hydrogen peroxide in the required presence of Fe.sup.2+. They, in effect, carried out a Fenton's reagent type reaction and then flocculated a portion of the remaining organic materials on the surface of the silica.
However, the prior art does not suggest adding sodium silicate solution or silica to a waste sample which contains cyanide ion in order to catalyze the oxidation of the cyanide by hydrogen peroxide.
Surprisingly, the present invention discloses that a source of silicon dioxide (e.g., sodium silicate solution or silica), when added to a waste sample which contains cyanide ion, catalyzes the oxidation of the cyanide by hydrogen peroxide. The cyanide is destroyed in a fraction of the time in comparison to oxidation without silicate and about equal to or less than the rate as when copper is added. Furthermore, the cyanide is unexpectedly more completely destroyed, i.e. to lower final concentrations of cyanide, than when silicate is not used, and in some cases even when copper is used.